Postfix changing Sender email when sending over an relayhost

[simo...@gmail.com]

Hi,

i have a configuration where i send my mails over a relaying host (mailgun). The relaying works but i have an issue with the emails. Normal the email should bei in the normal format mym...@mydomain.com but in my configuration the email is recived as mymail=mydoma...@mailservername.mydomain.com.
The email account logins are mym...@mydomain.com (Postfix with Mysql Database)

Can someone tell me how to fix this or where i can look after to get it fixed.

Thank you and best regards,
Alex